1. Field of the Invention
The present invention relates generally to liquid crystal display apparatuses and, more particularly, to a liquid crystal display apparatus which performs matrix driving by utilizing bistable conditions of a liquid crystal, particularly a chiral smectic liquid crystal or a ferroelectric liquid crystal.
2. Description of the Related Art
A kind of Liquid crystal display device is well known in which a liquid crystal mixture is packed between an electrode substrate having scanning electrodes and another electrode substrate having information electrodes, the scanning electrodes and the information electrode forming a matrix electrode structure, and in which a multiplicity of pixels are formed to display image information.
There are good hopes that display devices using chiral smectic liquid crystal, particularly a ferroelectric liquid crystal having bistability and high-speed response to an electric field will be put to practical use as a high-speed memory type display device. For example, such devices are disclosed in Japanese Patent Laid-Open Publication No. 107216/1981 and other documents. Many methods for driving such devices in a matrix drive manner have also been proposed. For example, practical drive methods are disclosed in Japanese Patent Laid-Open Publication No. 281233/1990 and other documents.
The conventional display devices using a ferroelectric liquid crystal, however, have a drawback in that during a long time period of standing in one stable state the threshold characteristic of the display device is changed by interaction at the interface between the substrate and the liquid crystal layer. To solve this problem, a drive method has been proposed which sets a rest period during which application of information signals to information electrodes is stopped. FIG. 1 shows waveforms of drive signals in accordance with this method. The waveforms shown in FIG. 1 represent waveforms C and D of image signals applied to information electrodes to display light and dark, a selection pulse having a width of .DELTA.T, auxiliary pulses having a width of 1/2.DELTA.T provided before and after the selection pulse and a rest period having a width of 1/2.DELTA.T and provided to separate auxiliary pulses with respect to time.
FIG. 2 is a graph showing the relationship between the rest period of the drive signals and a drive margin measured as a range in which the liquid crystal display device can suitably display information at a temperature of 10.degree. C., for example, when the drive conditions shown in FIG. 1 are V1=14.3 V, V2=14.3 V, V3=5.7 V, V4=5.7 V, V5=6.4 V, and VC=0 V. For improvement in the quality of an image displayed on the display device, a higher frame frequency is preferred. Accordingly, it is preferable to shorten the rest period. Considering both the drive margin and the frame frequency, it is suitable to set the rest period to a length about 1/2.DELTA.T. However, the optimal value of the rest period is between 1/2.DELTA.T and .DELTA.T.
FIG. 3 shows the drive waveforms when the rest period is 0.6 .DELTA.T. In this case, since the rest period is set to 0.6 .DELTA.T, one horizontal scanning period is formed of twenty six intervals and the information signals have .DELTA.T set as ten of these intervals and the rest period set as six of these intervals. If the rest period is selected so as to balance the drive margin and the frame frequency while considering the desired drive margin as described above, the number of intervals forming the drive waveform is considerably increased. Therefore, a need arises to operate the drive circuit at a higher speed and the rest period cannot be selected freely.